The present invention relates to an optical wavelength-division multi/demultiplexer for use in an optical communication such as wavelength-division multiplex communication.
Recently, a necessity of high-speed processing for a great amount of information has been increased in an optical communication. One method for satisfying such a necessity is an optical multiplex communication. However, the optical multiplex communication has not been sufficiently widely used under present conditions due to the fact that a laser diode (LD) as the source of multiwavelength light and an optical wavelength-division multi/demultiplexer which is an integral component of the optical multiplex communication are too expensive.
Japanese Patent Application Laying Open (KOKAI) No. 61-180209 discloses an exemplary optical wavelength-division multi-demultiplexer comprising a single-sided diffraction grating element. The diffraction grating element comprises a plane reflection plate which is formed at one side thereof with a plane-curved diffraction grating made of a plurality of grating grooves. More specifically, the plane-curved diffraction grating is formed so as to effect a focusing action and is coated with an Aluminium film so as to effect a reflection type diffraction. A light-input optical fiber for leading a light beam onto the diffraction grating element and a plurality of light-output optical fibers each for receiving a diffracted light beam are disposed so as to confront the diffraction grating grooves. The diffraction grating element and the optical fibers are mounted at predetermined positions on the support members.
In the above-mentioned construction of the optical wavelength-division multi/demultiplexer, a multiwavelength light beam output from the light-input optical fiber is incident on the diffraction grating at which the light beam is reflected at different reflection angles in accordance with the respective wavelengths, thereby being focused on the different positions. Namely, as the multiwavelength light beam is reflected by the diffraction grating, the light beam is divided into a plurality of diffracted beams having different wavelengths, respectively.
Such a wavelength-division multiplexing system is simple in construction. However, the above-mentioned plane-curved diffraction grating is the so-called in-line type holographic lens, so it is difficult to increase the diffraction efficiency thereof.
Further, in the above-mentioned wavelength-division multiplexing system, the incident multiwavelength light beam is divided by the single diffraction grating, so it is difficult to focus in sufficient a wideband multiwavelength light beam emitted from a light emitting diode (LED), resulting in an increase of the coupling loss of each of diffracted beams to be received into the corresponding light-output optical fiber due to an increase of the size of each of the beam spots.
If it is possible to use a LED as the source of multiwavelength light instead of a laser diode (LD), the wavelength-division multiplex communication will become widespread, because the LED is inexpensive. In actual conditions, however, it is impossible to use a LED as the source of multiwavelength lightt for the above-mentioned multi/demultiplexer due to the above-described increased coupling loss, and accordingly, there is nothing else than the use of an expensive LD.
More specifically, when such a conventional diffraction grating element having a single diffraction grating, i.e., a single-sided diffraction grating element, is used, it becomes necessary to decrease the grating periodic distance or pitch thereof so as to effect Bragg diffraction of light while decreasing the depth of the grating grooves so as to equalize the size thereof to that of the grating pitch, in order to increase diffraction efficiency. In this case, i.e., in the case of Bragg diffraction, however, the size of each diffracted beam spot becomes larger than that of the optical fiber for receiving the diffracted beam due to the increased grating pitch, resulting in an increase of the coupling loss of each diffracted beam. Contrarily, when the grating pitch is increased so as to cause Raman-Nath diffraction, it becomes possible to decrease the coupling loss of each of the diffracted beams in relation to the corresponding optical fiber. In this case, however, the diffraction efficiency of light is decreased due to the occurrence of highest-order diffracted light beams, resulting in an increased insertion loss of diffracted light on the whole in the wavelength-division multiplexing device. Essentially, those situation will not be improved even though the diffraction grating is formed so as to have a lens function, as shown in the above-mentioned reference.
In view of the above-described problems, an optical wavelenth-division multi/demultiplexer utilizing a double-sided diffraction grating element for effectively multiplexing or demultiplexing a wideband multiwavelength light beam emitted from a LED was proposed by the present applicant. The double-sided diffraction grating element is disposed between collimating and focusing lenses between light-input and light-output optical fibers. The double-sided diffraction grating element comprises a planar substrate which is provided at opposite sides thereof with diffraction gratings, respectively. The diffraction characteristics of the two diffraction gratings are such that the grating surfaces thereof as well as the grating directions are parallel to each other, but the grating pitches thereof are different from each other. In addition, the relationship between the light-input angle of the diffraction grating and the light-output angle of the diffraction grating is predetermined so as to have a certain difference angle.
According to the above-mentioned double-sided diffraction grating element, a light beam to be multiplexed or demultiplexed is passed through the two diffraction gratings, which can cause the deflection angle between the input and output light beams to be decreased, while maintaining an increased diffraction efficiency. Such a function could not be obtained by the above-described single-sided diffraction grating element having a single diffraction grating. This is because that in the case of diffraction grating element of light transmission type, although it is necessary to effect Bragg diffraction in order to increase the diffraction efficiency, as previously described, the deflection angle between input and output light beams is increased under the condition of such Bragg diffraction due to the increased angle of diffraction. Comparatively, in the case of the above-mentioned double-sided diffraction grating element having at opposite sides thereof two diffraction gratings, each of the diffraction gratings can effect Bragg diffraction with a high diffraction efficiency under the condition of increased angle of diffraction, and the combination of the two diffraction gratings enables the decrease of the deflection angle between input and output light beams while maintaining such a high diffraction efficiency. By the use of an optical wavelength-division multi/demultiplexer which can ensure a high diffraction efficiency and a low deflection angle, wavelength-division for a wideband multiwavelength light such as light emitted from a LED can be effectively performed without a significant increase of the diameter of spot of the diffracted beam.
Although such a multiplexing/demultiplexing system using the above-mentioned double-sided diffraction grating element can ensure a superior performance, the multiplexing/demultiplexing system has a disadvantage in that it requires collimate and focusing lenses other than the double-sided diffraction grating element, and in that the double-sided diffraction grating element occupies a relatively large space because the same is of a transmission type.